A reconstruction of the dinosaur Deinocheirus mirificus.
Photograph: Yuong-Nam Lee/KIGAM

Nearly 50 years after researchers uncovered the gigantic arms of a mysterious dinosaur in the Gobi desert, the true nature of the beast has finally been established.

Since its discovery in 1965, the only clues to the engimatic creature were its shoulders and forelimbs – the latter measuring an astounding 2.4 metres long – and a few ribs and vertebrae dug from the ground by a joint Polish-Mongolian expedition.

The fossils were extraordinary enough for scientists to declare the dinosaur a new genus and species. The name they decided upon was Deinocheirus mirificus, meaning “unusual horrible hand”.

In the absence of more complete remains, early reconstructions were at times wildly speculative. In 1970, one palaeontologist argued that Deinocheirus was a giant sloth-like climber that hung beneath the branches of enormous trees. A more accurate view put the dinosaur in a group of beaked omnivores called ornithomimosaurs, which resembled giant ostriches, at least superficially.

Deinocheirus walking. One of the newly discovered specimens was 11 metres long and weighed more than six tonnes. Video: Yuong-Nam Lee/KIGAM

But writing in the journal Nature on Wednesday , a Korean-led team of experts has transformed scientists’ understanding of the animal. They report the discovery of two nearly complete 70 million-year-old Deinocheirus skeletons, pieced together from fossils unearthed in Mongolia, along with a skull and hand that had been poached and sold on to private collectors.

With the new remains, the researchers built the first accurate reconstruction of the dinosaur. The creature stood tall on its back legs, but sported long, clawed forearms. Neural spines formed an impressive sail on its back and its long, toothless snout flared out to both sides. The duck-like bill may have helped Deinocheirus forage for food at the bottom of streams, while blunt, flattened bones under its claws prevented it from sinking on wet ground.

One of the new specimens grew to 11 metres long and weighed more than six tonnes. Its broad hips and large feet suggest it was not agile. The animal likely fed on plants and small animals, though the remains of fish were found among its stomach contents.

Writing in the journal, Young-Nam Lee at the Korea Institute of Geoscience and Mineral Resources, describes the team’s surprise on seeing the complete dinosaur. “The discovery of the original specimen almost half a century ago suggested that this was an unusual dinosaur, but did not prepare us for how distinctive Deinocheirus is – a true cautionary tale in predicting body forms from partial skeletons,” he says.